Starch compositions have been utilized in innumerable and diverse industrial applications, as for example, coatings, sizing agents, adhesives, etc. While such starches, often in derivatized or modified form, have a long history of utility in such applications, there is a continual need to meet broadening industrial demands imposed by new and modified processing means.
Starch based adhesives have been used extensively in the production of corrugated paperboard and corrugated containers. Conventional starch based corrugating adhesives include cooked or gelatinized starch and raw, i.e. unmodified starch particles. A resin, such as a urea formaldehyde, is often added to starch mixtures at low ratios, of approximately 1 part resin to 7 parts starch solids, to provide improved water resistance to paperboard or cardboard. It is known in the art to impregnate cardboard with urea formaldehyde resin to give crush resistance to the cardboard. Generally, however, the urea formaldehyde resin simply penetrates into the cardboard and does not act as an adhesive. The use of starch alone without a resin will provide some adhesive property but no water resistance or crush strength.
"Dual arch" is a term in the art which describes and refers to the laminating of two sheets of paperboard, such as Kraft medium, used in constructing corrugated board. The laminated sheet is then fluted and bonded or glued to two sections of liner board. This process of dual arch lamination increases the crush strength of the corrugated board.
It is known in the art to bond the two sheets of the dual arch mediums to the liner boards with polyvinyl acetate or starch based adhesives. Polyvinyl acetate (PVA) does give some degree of strength but the level of strength improvement is low and, for many applications, inadequate. PVA can also provide some water resistance to the resulting corrugated board however, a major disadvantage of PVA is relative high cost. Starch based adhesives, although lower in cost, will not add substantially to crush strength and have little water resistance.
U.S. Pat. No. 4,623,412 issued on Nov. 18, 1986 to Bohme et al. describes a two step process in which a mixture of urea-formaldehyde, a catalyst and a wetting agent is applied to liner board to impregnate the liner board. In a second step, an aqueous primer, which may be a starch solution, is applied. The primer causes the resin to impregnate the liner board and to leave the surface relatively resin-free.
Bohme et al. also describes a process for impregnating the liner and medium for making corrugated board. Bohme et al. impregnate the board in line with a mixture of urea-formaldehyde resin, wetting agent and a catalyst. After the board is impregnated, the board can be primed with a mixture of water and starch. The board is then dried, remoistened and passed on to a corrugator.
U.S. Pat. No. 3,487,033, issued Dec. 30, 1969 to McElmary and U.S. Pat. No. 4,379,015, issued Apr. 5, 1983 to Ware et al., describe corrugating adhesive compositions made from urea-formaldehyde, starch and water. The McElmary adhesive is formulated of 2 parts to 25 parts of urea-formaldehyde resin to 100 parts of starch solids. Therefore, the ratio of resin solid to starch solids, can be as high as 1:4. The Ware et al. adhesive can be formulated with as much as 1 part of resin solids to 9 parts of starch solids.
U.S. Pat. No. 4,400,480, issued Aug. 23, 1983 to Silano et al., and U.S. Pat. No. 4,424,291, issued Jan. 3, 1984 to Leake et. al., describe corrugating adhesives that comprise resin, starch and water. The Leake et al. adhesive may have a resin solids to starch solids ratio as high as 98:2. The resin must be a resin derived from ethylenically unsaturated carboxylic acids such as acrylic acid and the like. The Silano et al. adhesive may have a resin solids to starch solids ratio as high as 1:10. The resin is a crosslinking additive prepared by reacting acetone and formaldehyde with the further addition of dihydroxyethylene urea.
The Bruner et al. patent U.S. Pat. No. 3,485,776, issued Dec. 23, 1969, describes a modified starch composition useful as a binder for non-woven fabrics. The composition is made by reacting dry granular corn starch and urea-formaldehyde resin in water. The starch swells as it reacts. The proportion of resin to starch falls within the range of 0.6:1 to 2:1 on a weight percent basis.
A Japanese patent, No. JP057392, dated Nov. 5, 1979, issued to Aika Kogyo KK, describes a water resistant adhesive for corrugated cardboard that may be prepared from acetone-formaldehyde resin, urea resin, or urea-melamine resin or melamine resin, and alkali starch paste. The ratio of total resin solids to starch solids can be as high as 40:100 by weight.
The Rumberger patent, U.S. Pat. No. 3,293,057, issued Dec. 20, 1966, describes a modified starch composition comprising starch, urea and polyfunctional aldehyde. However, in Rumberger the aldehyde content does not exceed 20%, by weight, of the composition. The Shoenberger et al. patent, U.S. Pat. No. 3,355,307, describes a corrugating adhesive comprising starch, but which does not utilize an aldehyde resin. Schoenberger et al. describes an ungelatinized starch which is chemically treated so as to act as its own carrier whereby a corrugating adhesive is produced. However, no urea-formaldehyde resin is utilized.
The Black patent, U.S. Pat. No. 3,697,355, issued Oct. 10, 1972, describes a urea-aldehyde resin adhesive used to bond cellulosic particles. The adhesive contains no starch.
The Black patent, U.S. Pat. No. 3,905,847, issued Sept. 16, 1975, describes a urea-aldehyde resin adhesive for bonding wood flakes comprising urea-formaldehyde resin, sodium chloride and magnesium chloride. The adhesive contains no starch.
U.S. Pat. No. 4,018,957, issued Apr. 19, 1977 to Demko et al., teaches the preparation of corrugating adhesive compositions comprising ethylenically unsaturated polymers, ungelatinized starch, a urea-formaldehyde resin, an acid metal salt, and water. Demko et al. requires the presence in the compositions of crosslinkable unsaturated polymers.
Similarly, U.S. Pat. No. 3,984,225, issued Oct. 5, 1976 to Hofmann et al., teaches the preparation of corrugating adhesive compositions comprising starch, PVA, a copolymer of vinylacetate and ethylene and, optionally, a water resistant thermosetting resin such as urea-formaldehyde. The present invention does not require ethylenically unsaturated polymers or PVA or copolymer of vinylacetate and ethylene.
The Krankkala patent, U.S. Pat. No. 4,396,453, issued Aug. 2, 1983, describes a starch-based adhesive which comprises water, starch, a carboxylated styrene-butadiene latex, a crosslinker and thickener. In one embodiment the crosslinker may be a mixture of glyoxal and urea-formaldehyde. However, the crosslinker is present at low levels (0.5 to about 5 weight percent) based upon total weight of the final adhesive.
No art is known to the inventors describing a curable, paper corrugating adhesive made of an aldehyde resin, such as urea-formaldehyde copolymer resin, and starch, a curing agent and water, wherein the adhesive has as high a ratio of resin solids to starch solids as the adhesive of this invention. The highest ratio of urea-formaldehyde resin to starch described in the known corrugating adhesive patent literature was 2:1 (Bruner et al. U.S. Pat. No. 3,485,776). Bruner et al., however, is directed toward binders for non-woven fabrics and not for laminating adhesives or saturants for corrugated paperboard or paper core winding as in the present invention.
No art is known to the inventors that describes an adhesive made specifically for saturating and adhering two plies which form a dual arch corrugated paperboard.
Therefore, it is an advance in the art to provide an aldehyde starch composition which is functional both as a saturant and as a laminating adhesive and which contains a high ratio of resin to starch.
Thus it is an object of the present invention to provide a novel, curable aldehyde resin and starch composition which will act as a paper saturant, provide improved crush strength, provide moisture resistance, and reduce the cost to produce high performance corrugated dual arch paperboard.